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Seifertite, a dense orthorhombic polymorph of silica from the Martian meteorites Shergotty and Zagami

¹ Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
² Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. 20015, USA
³ School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85069, USA
⁴ University of Arizona, Tucson, AZ, 85721, USA
⁵ Insitute of Geochemistry, Chinese Academy of Sciences, Guangzhou, PR China
⁶ Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA

^* Corresponding author, e-mail: ahmed.elgoresy{at}uni-bayreuth.de

Seifertite is a dense orthorhombic polymorph of silica with the scrutinyite (-PbO₂) type structure that was found as lamellae occurring together with dense silica glass lamellae in composite silica grains in the heavily shocked Martian meteorite Shergotty. The mineral is also intergrown in some grains with minor stishovite and a new unnamed monoclinic dense silica polymorph with a ZrO₂-type structure. Seifertite has also been found in the Martian shergottite Zagami and is a minor constituent in other Martian shergottites. Chemical analyses of seifertite in Shergotty indicate major SiO₂ with minor concentrations of Al₂O₃ and Na₂O. Selected-area electron diffraction (SAED) and X-ray diffraction can be interpreted in terms of an orthorhombic pattern from a scrutinyite (-PbO₂) structure. The cell parameters are a = 4.097(1) Å, b = 5.0462(9) Å, c = 4.4946(8) Å, V = 92.92 ų, Z = 4, and the space group is Pbcn or Pb2n. Density is (calc.) = 4.294 g/cm³ (with pure SiO₂), 4.309 g/cm³ (with empirical formula). It is inferred that seifertite was formed by shock-induced solid-state transformation of either tridymite or cristobalite on Mars at an estimated minimum equilibrium shock pressure in excess of 35 GPa. The new mineral is named after Friedrich A. Seifert (b. 1941), founding Director of the Bayerisches Geoinstitut, Universität Bayreuth, Germany, for his seminal contributions to high-pressure geoscience.

Key-words: seifertite, new mineral, silica, high-pressure phases, -PbO₂ structure type, shock metamorphism, Shergotty meteorite.